1. Field of Invention
This invention relates to mechanisms for providing linear snap action. More particularly, it relates to such mechanisms which provide snap action and develop the mechanical forces required to rapidly make and break the contacts in electrical switches. It has particular application as an operating mechanism for isolating switches in medium voltage motor starters.
2. Background Information
Electrical switches used for connecting and disconnecting electrical components from medium voltage power sources require a mechanism for developing the high mechanical forces required to open and close the switch contacts.
An example of such a switch is the isolating switch in a motor starter for medium voltage (2300-7200 volt) motors. A motor starter incorporating such an isolating switch is disclosed in U.S. Pat. No. 4,086,452. This starter includes a contactor which performs the normal switching functions of the starter and fuses for overcurrent protection. The isolating switch is provided to remove power from the line side of the components in the starter unit for maintenance.
The isolating switch makes and breaks contact between a set of three-phase moveable stab contacts and the three-phase line terminal assemblies. The stab contacts are mounted on a sliding tray and are integral with fuse clips gripping one end of power fuses which are pivoted by movement of the tray. The tray is advanced to close the contacts of the isolating switch and retracted to open them by a handle on the outside of the starter unit through a connecting rod. Considerable mechanical force is required to accelerate the mass of the tray, the attached stab contacts, and the pivoted power fuses and to engage and disengage the stab contacts in the line terminal assemblies. While the main current is disconnected by the contactor before the isolating switch is operated, some current is drawn through the isolating switch with the contactor open to provide primary exitation for the control transformer supplying power to the control circuits for the contactor, and for a metering transformer which monitors voltage and other parameters for performing the overload function. However, the isolating switch is not provided with arc extinguishers. It can be appreciated therefore, that sizeable mechanical forces must be developed to operate the isolating switch, and that the switch must be operated rapidly to avoid damage due to arcing.
Snap acting mechanisms have been used elsewhere to rapidly generate sizeable mechanical forces. However, these devices commonly utilize an over center toggle, or rotary motions or a combination of both. Space limitations in the starter unit preclude use of such devices in this application.
There is a need therefore for an improved operating mechanism for electrical switches which can generate the sizeable mechanical forces required to operate the switch, with rapid engagement and disengagement of the electrical contacts.
More particularly, there is a need for an improved snap action mechanism for operating electrical switches.
There is a further need for such mechanisms which are compact and which preferably utilize linear motion.
There is a specific need for such a mechanism for the isolating switch for medium voltage motor starter.
There is a subordinate need for such a mechanism which is simple and reliable, and can be readily adapted to existing isolating switches.